Aerial delivery is a means frequently used for transporting cargo quickly to areas of limited or hostile access, without the availability of any nearby airports. At times, people in isolated areas, such as jungles, deserts, mountains, polar regions, or combat zones, are in need of essential supplies, including food and medical supplies, but are not within access to an airport at which a supply plane could land. In these instances, aerial delivery of cargo from in-flight aircraft is the sole option. These aerial delivery systems involve the use of parachute systems to slow the descent and gently land the cargo platforms on the ground.
Similar parachute systems are further utilized for recovery of aeronautical and astronautical vehicles, including rocket boosters, experimental aircraft and space capsules, returning back to the earth's surface from flights in the upper atmosphere or outer space.
In these aerial delivery or recovery systems, a series of parachute deployments is often used to progressively slow the descent velocity of the payload. Use of a series of parachute deployments is often necessary because the force and impulse on the suspension lines and straps of a main parachute (i.e., one large enough to slow the cargo platform to an acceptable landing velocity) deploying at the terminal, free-fall velocity of the cargo platform would be excessive, causing the parachute system to fail. Instead, the cargo platform is typically slowed in a series of stages using subsequently larger parachutes.
A drogue parachute is typically deployed first from the parachute system. The drogue is a small parachute which can be easily deployed from its container by a tether attached to the launching cargo plane or by an easily deployed stored energy means such as a spring launched pilot parachute. As the drogue parachute is deployed and inflated, it moderately decelerates the suspended cargo platform, as well as orientating the cargo platform into its desired upright attitude, without excessive strain on the slings and parachute canopy from which the cargo platform is suspended. After a predetermined time period, the drogue parachute is released from the suspended cargo platform. The drag provided by the released drogue parachute is then utilized to pull and deploy the next, larger parachute. This next parachute may be the final, main parachute, or another intermediary parachute prior to another subsequent deployment of the main parachute, depending upon the size of the cargo platform and the design of the parachute system.
To accomplish this release design, the drogue parachute is attached to the suspended cargo platform by a release mechanism. The suspended cargo platform is suspended from the release mechanism typically by a series of suspension slings. The suspension slings maintain the suspended cargo platform in a stable, level attitude. The number of suspension slings is typically four, with one routed to each corner of a square or rectangular cargo platform supporting the suspended cargo. For larger platforms, a greater number of suspension slings may be used. The suspension slings converge at a point above the suspended load to the release mechanism, located at the apex of the pyramid formed by the suspension slings.
The drogue parachute may be attached to the release mechanism by a single sling or riser. From the top end of this sling, a number of suspension lines radiate to the perimeter of the drogue parachute canopy.
One common release mechanism is a pyrotechnic cord cutter powered by an explosive or pyrotechnic charge. This mechanism utilizes the detonation of a small explosive charge to drive a cutting blade through the suspension sling. For example, Norton, U.S. Pat. No. 4,493,240, disclosed a pyrotechnic cord cutter comprising an elongated cylindrical body with a lateral aperture proximate to one end of the body, through which passes the suspension line or other support line in the parachute system. A chamber is located at the other end of the body, containing an explosive charge. An opening is provided in the medial end of the chamber, into which is registered the rear end of a cutting blade. Upon detonation of the explosive charge in the chamber, the cutting blade is propelled through the aperture, severing the cord within.
The pyrotechnic cord cutters are typically usable only for relatively small-diameter cords. As the weight of a platform and payload increases, the size of the cord or strap between the parachute and payload platform increases. A pyrotechnic cord cutter for payloads above a moderate size becomes too large for practical handling and would incorporate an explosive charge too large and powerful for safe handling by personnel.